This application claims the benefit of Japanese patent application no. 2001-094838, filed Mar. 29, 2001, incorporated herein by reference.
The present invention relates to the image coding equipment and the image coding programs that code picture signals so that the coding result should not exceed restricted transmission bit rates, especially to such equipment and such programs that improve the quality of an image in specific domains without generating the sense of incongruity in the entire picture.
In recent years, it has become possible to transmit and receive picture signals through a transmission way or to store picture signals to storage media owing to the development of information communication technology and the maintenance of infrastructures.
In order to transmit picture signals through a transmission way with restricted transmission bit rates or to store picture signals to the storage media with restricted capacities the technology of picture signals, which carries out the compression coding of picture signals, is indispensable since the amount of information of picture signals is generally large.
There is MPEG (Moving Picture Expert Group) the standardization of which ISO/IEC is advancing as one of the compression coding methods of picture signals. This is the technology to transmit or store coded picture signals satisfying the bit rates of transmission ways or the capacities of storage media while cutting down the redundancy of information using time correlation and spatial correlation of picture signals.
In compression coding of picture signals, however, because picture signals are coded so that the coding result satisfy the restricted bit rate, the amount of code assigned to each frame is restricted and the quality of picture image becomes deteriorated. Especially when pictures in a screen are coded uniformly, the picture image quality becomes uniformly deteriorated.
Take for example, the case of a picture signal coding for a TV conference (a tele-conference). Though users want the quality of facial portion of the image to be better than that of the background portion, when the picture signals in a screen are uniformly coded the quality of the face portion, which is important, and that of the background portion, which is not important, become the same and the picture image after decoding does not satisfy the subjective quality of the image reflecting the intention of the users.
Then, the method that makes the quality of image of specific domains better than other domains is proposed as a technology in which picture signal coding is done reflecting the intention of users.
For example, in order to make the quality of image of specific domains better than others, changing coding parameters in the inside and outside of specific domains is proposed.
By the method indicated by ISO/IEC JTC1/SC29/WG11 MPEG 95/030, a domain (e.g., region) is chosen from a video image and the quality of image of a selected domain is made better than other domains by changing quantization values.
Moreover, with the picture transmission system and picture coding equipment which are indicated by laying open for public inspection JP 11-146395 A by the Commissioner of the Japan Patent Office (it is hereafter called the conventional example), the quality of image of a selected domain is made improved by the user's selection of a domain and by applying quantization matrices, which are prepared beforehand, to the selected domain.
However, if coding by simply using different coding parameters in the inside and the outside of a specific domain, a sense of incongruity will occur in the entire picture because the difference of the quality of the image between domains is large and the boundary of domains is conspicuous.
Below, the technique of the conventional example is explained.
FIG. 14 shows the block diagram of the conventional example.
In FIG. 14, the picture coding equipment (1401) is configured with the subtraction section (1408) which is connected to the image input section (1420), the DCT section (1402), the quantization section (1403), the VLC section (1404), the de-quantization section (1405), the inverse DCT section (1406), the addition section (1409), the frame memory (1407), the bit amount control section (1413) linked to the domain input section (1410), and the selection section (1412), quantization table buffer (1411), and is linked to the output section (1414).
The operation is explained below about the image coding equipment constituted as mentioned above.
The image input section (1420) outputs picture signals to the subtraction section (1408) for every frame.
The domain input section (1410) is a section that selects domains of the image quality that should be improved, and outputs the information on selected domains to the bit amount control section (1413). It may be case that one domain is selected and that its information is output to the bit control section.
Using the amount of generated codes on the previous frame being inputted from the VLC section (1404), the bit amount control section (1413) computes the amount of codes allocated to the inside and the outside of the selected domains of the present frame, and outputs it to the selection section (1412).
Based on the amount of codes allocated to the inside and the outside of the selected domains which the bit amount control section (1413) computed, the selection section (1412) selects the quantization tables on which two or more quantization matrices were recorded respectively corresponding to the inside and the outside of the selected domains, and inputs each selected quantization table into a quantization section.
The subtraction section (1408) subtracts the prediction picture inputted from a frame memory (1407) from the original picture image input from the image input section (1420), and outputs a prediction error to the DCT section (1402).
The DCT section (1402) applies DCT transformation on the prediction error input from the subtraction section (1408), and outputs the computed DCT coefficients to the quantization section (1403).
The quantization section (1403) quantizes using the quantization tables input from the selection part (1402) the DCT coefficients input from the DCT section (1402) and outputs the quantization tables and the quantized DCT coefficients to the inverse quantization section (1405) and the VLC section (1404).
The VLC section (1404) outputs the amount of generated codes to the bit amount control section (1413) while it carries out the variable length coding of the quantization tables inputted from the quantization section (1403), and the quantized DCT coefficients and outputs the variable length codes to the output part 1414.
The de-quantization section (1405) de-quantizes using the quantization tables input from the quantization section (1403) the quantized DCT coefficients, and outputs the de-quantized DCT coefficients to the inverse DCT section (1406).
The inverse DCT section (1406) applies the inverse DCT transformation to the de-quantized DCT coefficients input from the de-quantization section (1405), and outputs the decoded prediction error to the addition section (1409).
The addition section (1409) adds the prediction error input from the inverse DCT section (1406) and the decoded picture of the previous frame being input from the frame memory (1407), and computes the present decoded picture, and outputs it to the frame memory (1407).
The frame memory (1407) updates the internal buffer using the decoded pictures input from the addition section (1409), and outputs the decoded pictures to the subtraction section (1408) and the addition section (1409).
As mentioned above, the quality of image of selected domains can be improved by changing the quantization tables of the inside and the outside of the selected domains.
When the quality of images of selected domains is improved by using different quantization matrices for the selected domains and other domains as is done in the conventional example, block noises will arise because of the difference of quantization values, and the picture will give the sense of incongruity in the entire picture.
This is the same for other conventional technology such as coding pictures by using different quantization values thus generating a sense of incongruity in the entire picture.